Ohio State discovery reveals flaw that may shorten lithium-ion battery life

Just because winter break is coming up for many students doesn't mean it's a bad time for a chemistry lesson. This one comes courtesy of Ohio State University and poses new questions about the functional lifetime of a typical lithium-ion battery as used in a hybrid or battery electric vehicle.

A couple of Buckeye engineers found that, over time, a battery's lithium accumulates on the sheet of copper that serves as a "current collector" that transfers energy. With lithium ions traveling between the anode and cathode, lithium was previously thought to collect only on the anode's surface as the battery ages.

The issue of battery life and energy retention has been a major one as automakers offering electric-drive vehicles need to assure potential buyers that the newfangled car's battery life will equal or exceed the timeframe of typical vehicle ownership. General Motors gives an eight-year, 100,000-mile warranty on the batteries in the Chevrolet Volt extended-range plug-in (longer in California), while Nissan has said the battery pack for the all-electric Leaf will have 80 percent capacity after five years and 70 percent after a decade. Nonetheless, Nissan has been getting flak from a group of Leaf owners in Arizona who said their cars' battery capacity was shrinking more quickly than advertised. Perhaps OSU's research will help future electric vehicles avoid similar complaints.

Study Reveals New Factor that could Limit the Life of Hybrid and Electric Car Batteries

COLUMBUS, Ohio – A new study of the batteries commonly used in hybrid and electric-only cars has revealed an unexpected factor that could limit the performance of batteries currently on the road.

Researchers led by Ohio State University engineers examined used car batteries and discovered that over time lithium accumulates beyond the battery electrodes – in the "current collector," a sheet of copper which facilitates electron transfer between the electrodes and the car's electrical system.

This knowledge could aid in improving design and performance of batteries, explained Bharat Bhushan, Ohio Eminent Scholar and the Howard D. Winbigler Professor of Mechanical Engineering.

"Our study shows that the copper current collector plays a role in the performance of the battery," he said.

The study, which appears in a recent issue of the journal Scripta Materialia, reflects an ongoing collaboration between Bhushan and Suresh Babu, professor of materials science and engineering and director of the National Science Foundation Center for Integrative Materials Joining for Energy Applications, headquartered at the university. The team is trying to determine the factors that limit battery life.

Lithium-ion batteries are the rechargeable batteries used in most hybrid-electric cars and all-electric cars as well. Inside, lithium ions shuttle back and forth between the anode and cathode of the battery – to the anode when the battery is charging, and back to the cathode when the battery is discharging.

Previously, the researchers determined that, during aging of the battery, cyclable lithium permanently builds up on the surface of the anode, and the battery loses charge capacity.

This latest study revealed that lithium migrates through the anode to build up on the copper current collector as well.

"Our study shows that the copper current collector plays a role in the performance of the battery."

"We didn't set out to find lithium in the current collector, so you could say we accidentally discovered it, and how it got there is a bit of a mystery. As far as we know, nobody has ever expected active lithium to migrate inside the current collector," Bhushan said.

Shrikant Nagpure, now postdoctoral researcher at Ohio State, carried out this research as a part of his doctoral degree. He examined batteries that were aged in collaboration with the university's Center for Automotive Research, where colleagues Yann Guezennec and Giorgio Rizzoni have studied battery aging for several years, in collaboration with the automotive industry.

Key to the discovery of lithium in the current collector was collaboration between the Ohio State team and Gregory Downing, a research chemist at the National Institute of Standards and Technology and an expert on a technique called neutron depth profiling (NDP), a tool for impurity analysis in materials.

Previously, the researchers used NDP to study the cathodes and anodes of six off-the-shelf lithium-ion car batteries – one new battery and five batteries which they aged themselves in the laboratory – and found that lithium builds up on the anode surface over time.

To understand more about how these batteries degrade, Bhushan and his colleagues have been studying the batteries further, at various scales ranging from the millimeter (thousandths of a meter) down to the nanometer (billionths of a meter) with different techniques.

In the NDP technique, researchers pass neutrons through a material and capture the charged particles that emerge from the fission reaction between neutrons and lithium in the electrodes. Since different chemical elements emit a certain signature set of particles with specific energies, NDP can reveal the presence of impurities in a material.

In this latest study, NDP detected the presence of lithium in the copper current collector from one of the aged batteries. The detection was measured as a ratio of the number of copper atoms in the collector to the number of lithium atoms that had collected there. The test yielded a ratio of up to 0.08 percent, or approximately one lithium atom per 1250 copper atoms in the collector.

That's a small number, but high enough that it could conceivably affect the electrical performance of the current collector – and, in turn, the performance of a battery, Bhushan said. He hopes that battery makers will further investigate this phenomenon and use the information to design new materials that might prevent lithium from escaping the electrode material.

Next, he and his colleagues will study the impedance, or internal electrical resistance, of lithium-ion batteries on the nanoscale.

Funding for this study came from the Institute for Materials Research at Ohio State.

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Picky point. GM warrants their battery for 8 years, 100k miles, 80% range. Nissan does not. They say their battery should be 70% under normal conditions after 10years. Nissan also stated that all LEAFs returned after their lease was up would get new batteries before being sold as used cars.

If they had an estimate of the concentration profile of lithium in the current collector, an estimate of the amount of lithium that was "lost" to the anode current collector could be made and that could be correlated to the effective capacity loss to determine if in fact this was a real issue. It may be insignificant. Check me on this, but I thought the capacity loss problem was primarily on the anode solid/electrolyte inface, and in the cathode. When deep cycled, the cathode has to give up nearly all of the lithium and then can at somewhat elevated temperatures undergo mild sintering-like effects that make it more difficult for the cathode to accept the same amount of lithium (lower energy) or, alternatively, lithium is lost to the interface layer that forms on the anode, and is known to eat up a relatively large amount of lithium, and the layer itself has a higher resistance and therfore more energy is lost to heat. So, deep discharge kills the cathode, and high current causes faster reaction on the anode (this includes fast charging which cause faster capacity decline). So, if you beat your electric car and discharge your battery to near zero, you fade the battery faster. Add a little extra temperature and you assist these degradation mechanisms even further. I'll bet the guys with problems in AZ are leadfoots with long commutes.

We are early in the stage of inventing traction batteries and with failures comes additional knowledge about using the device in the real environment. Hopefully, this knowledge will lead to an even better, cheaper battery. Early battery degradation would not be a problem if it was not so dang expensive to replace; the battery could be swapped out when it 's capacity fell below what was acceptable to the owner. In fact, I see a future aftermarket business, undercutting dealers, that will sell and install rebuilt upgraded batteries. For example; the Leaf's current battery is assembled from modules that are rated at an energy density of 140 watt hours per kilogram. When a module become available with a higher density(longer range) and the price is right, that might be a time to consider a swap.

The researchers aged the battery in the lab, then tested them for Li in the Cu collector, finding out there is .08% Li in the collector. Missing in the report is the equivalent age of the lab induced battery as compared to a typical battery used in the consumer's EV.

Irrelevant. This research simply discovered that lithium plating was not limited to the graphite surface but also migrated through to the copper current collector. We already knew that lithium plating occurs

I'd say it's safe to assume they used a realistic timescale. Also, the .08% number relates to the amount of lithium to copper, but what is also important, and not mentioned, is what percentage of total lithium is now locked up and unavailable for energy transfer.

What is a "realistic timescale" for EV research is still subjective; there is no standard, yet. You have no basis for your assumption. As to your comment regarding "percentage of total lithium is now locked up and unavailable for energy transfer," that is irrelevant. That is not the issue here. The issue is your claim that the lab induced aging of the Li ion battery and its equivalent age in a consumer EV is irrelevant. As to your attempt to deflect the issue, Good try.

Its hard to digest this (even with a Chemistry degree under ones belt). "Lithium" chemistries vary, the electrodes potentially more so, and then there are the external variables of heat and cooling, and the pattern of loads and chargeing that the battery experiences over time. Ultimately, while the chemistry is very interesting (to me) the only thing that really matters to the consumer is the total of the engineering reflected in the range of the car as it ages.... which is of course only simulated in the automakers battery labs and reflected by their warrantees.